Abstract
Light and temperature are crucial factors for the annual growth rhythm of tree seedlings of the boreal and temperate zone. Dormant, vegetative winter buds are formed under short days (SD) and altered light quality. In the conifer Norway spruce, expression of FTL2 increases and PaCOL1-2 and PaSOC1 decrease under light regimes, inducing bud set. Although temperature is known to modulate the timing of bud set, information about combined effects of light climate and temperature on bud phenology and gene expression is limited. We studied the interactive effects of temperature (18, 22/24 °C) and day extension with blue (B), red (R) or far-red (FR) light or different R:FR ratios compared to SD on growth–dormancy cycling and expression of FTL2, PaCOL1-2 and PaSOC1 in Norway spruce seedlings. Day-extension with B light and all treatments involving FR light sustained shoot elongation, with increased growth at higher temperature. The R light treatment resulted in delayed/prevented bud set compared to SD, with more delay/prevented bud set at 24 °C than 18 °C. This was associated with lower PaFTL2-transcript levels at 24 °C and more rapid subsequent bud burst. For the growth-sustaining treatments (long days, FR and B light), the PaFTL2-transcript levels were generally lower and those of PaCO1-2 and PaSOC1 higher compared with SD and R light. In conclusion, our results demonstrate more reduced/prevented bud set and faster bud burst with increased temperature under day extension with R light, indicating less deep dormancy than at lower temperature. Also, sustained shoot elongation under the B light treatment (27 µmol m−2 s−1) in contrast to the lower B light-irradiances tested previously (≤13 µmol m−2 s−1), demonstrates an irradiance-dependent effect of day extension with B light.
Highlights
IntroductionThere is growing interest in efficient year-round production of high-quality forest tree seedlings under semi-controlled or controlled conditions prior to planting in the field
Terminal bud set under short days (SD) was significantly affected by temperature (p ≤ 0.0001) and provenance (p ≤ 0.001), and there was a significant interaction between temperature and provenance (p ≤ 0.01; Table S1b)
For growth and terminal bud development as well as subsequent bud burst, the results indicate an interplay between temperature and light parameters in terms of photoperiod and light quality used for day-extension treatments
Summary
There is growing interest in efficient year-round production of high-quality forest tree seedlings under semi-controlled or controlled conditions prior to planting in the field. Recent years’ developments in light-emitting diode (LED) technology have increased the efforts in tailoring the light climate for production of plants with desirable physiological traits. The production of high-quality forest tree seedlings using such lighting systems requires improved knowledge about the responses of the plants to the light and temperature climate during the growth–dormancy cycling. The critical photoperiod is well known to increase with higher northern latitude of origin [1,2,3,4,5]
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